Image forming apparatus and control method therefor

ABSTRACT

An image forming apparatus image which is capable of accurately correcting the magnification of an image without degrading the quality of the formed image or reducing productivity, to thereby make it possible to cope with an increase in the operating speed of the apparatus. The image forming apparatus is comprised of an image processing section having an image area-separating section, and an image forming section having an exposure control section. The image area-separating section separates image data into at least two kinds of areas. The exposure control section performs magnification correction on at least one area of the image data separated by the image area-separating unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus thatperforms image formation by an electrophotographic method, and a controlmethod therefor.

2. Description of the Related Art

Conventionally, in the field of an electrophotographic image formingapparatus that performs image formation by executing the steps ofcharging, exposing, developing, transferring, and fixing, a technique(heat fixing method) is known in which an unfixed toner imagetransferred onto a recording sheet is heated and fused for fixing. Theheat fixing method include not only a sheet heater method and aninfrared lamp method, but also a fixing roller method, which iscurrently most widely used, and an IH (Introduction Heating) fixingmethod.

In an image forming apparatus that fixes a toner image on a recordingsheet by the above heat fixing method, the recording sheet shrinks afterthe toner image has been fixed, since moisture contained in therecording sheet is evaporated by heat during heat fixation. Therecording sheet for use is generally different in characteristics andproperties, depending on the use (one-side printing or both-sidedprinting), conditions (humidity and temperature) in which the apparatusis used, thickness, hardness, and the maker which made the sheet. Theratio of shrinkage (shrinkage ratio) is different depending on the kindand thickness of the recording sheet. It is empirically known that oncethe recording sheet shrinks due to heat fixation, it takes apredetermined time period (approximately 15 to 20 minutes) for therecording sheet to absorb moisture in the air to return to its originalsize.

In general, however, when images are formed on both sides of a recordingsheet, a toner image is transferred onto one surface (front surface) ofthe recording sheet for heat fixation, whereafter another toner image istransferred onto the other surface (back surface) of the recordingsheet, and then is heat-fixed again. Therefore, when the other tonerimage is transferred onto the back surface of the sheet after the tonerimage is transferred onto the front surface thereof, the other tonerimage is transferred onto the sheet in a shrunk state. This causes theproblem that the images on the front surface and back surface of therecording sheet are different in size. Therefore, it is required toadjust the position of the recording sheet for image formation thereonwith high accuracy according to the amount of heat-caused shrinkage. Tothis end, it is a general practice to change the magnification of theimage according to the change in the size of the recording sheet. Forexample, there has been proposed an invention in which the imagemagnification is changed by controlling the rotational speed of thescanner motor, to thereby correct displacement of the position of thesheet for image formation.

For example, Japanese Laid-Open Patent Publication (Kokai) No.H04-288560 and Japanese Laid-Open Patent Publication (Kokai) H10-149057proposes a technique of detecting vertical size and horizontal size of arecording sheet by an optical sensor in a conveying path, calculatingthe vertical and horizontal expansion/contraction ratios of therecording sheet based on results of the detection, and controlling thescanning speed of a scanning optical system based on theexpansion/contraction ratios of the recording sheet to thereby changemagnification of the image.

Further, Japanese Laid-Open Patent Publication (Kokai) No. 2004-351908,for example, discloses a technique of controlling the length betweenimage data by adding an image clock for use in transferring the imagedata at a desired point to thereby perform the magnification correctionin a main scanning direction (perpendicular to the image readingdirection and is along the longitudinal direction of the photosensitivemember as an image carrier), to thereby correct the image size of animage to be printed.

According to the above-described prior art, the magnification of animage to be formed is changed by switching the rotational speed of thescanner motor, and hence the rotational speed of the scanner motor ischanged at intervals between adjacent pages (intervals between adjacentrecording sheets) to thereby perform image formation on a page-by-pagebasis. In this case, during changing of the rotational speed of thescanner motor (during switching the speed), the image formation cannotbe started, and hence a next image formation cannot be executed until achange in the rotational speed is completed (the speed has beenswitched). Although depending on the type of the scanner motor, forexample, it takes about several hundred msec. to complete changing ofthe speed after starting the same. When the scanning speed of thescanning optical system of the image forming apparatus is switched, ittakes long time before the scanning speed becomes stable after it ischanged. This causes a reduced print speed to degrade productivity ofthe image forming apparatus, when a sheet-to-sheet interval (conveyingintervals of sheets which are conveyed to the transfer position) isshort. Further, it is necessary to control the sheet-to-sheet intervalsuch that a time period taken before the scanning speed becomes stableis ensured, which makes it impossible to cope with an increase in theoperating speed of the image forming apparatus.

Further, a change in the image clock for transferring the image data iscan only corrects the magnification of the image data in a main scanningdirection (direction orthogonal to the image reading direction).Therefore, to cope with correction of the magnification in a subscanning direction (image reading direction), it is necessary to combinethe above-described technique with the corrections of the magnificationdisclosed in Japanese Laid-Open Patent Publication (Kokai) No.H04-288560, and Japanese Laid-Open Patent Publication (Kokai) No.H10-149057. In this case, the rotational speed of the scanner motor isswitched, and hence, similarly to the above, there arises the problemthat the productivity is lowered due to a decreased in the print speed,and it is impossible to cope with an increase in the operating speed ofthe image forming apparatus.

SUMMARY OF THE INVENTION

The present invention provides an image forming apparatus which iscapable of accurately correcting the magnification of an image withoutdegrading the quality of the formed image or reducing productivity, tothereby make it possible to cope with an increase in the operating speedof the apparatus, and a control method therefor.

In a first aspect of the present invention, there is provided an imageforming apparatus that performs image formation by transferring an imageformed on an image carrier based on image data onto a recording medium,and heat-fixing the image thereon, comprising an image area-separatingunit configured to separate the image data into at least two kinds ofareas, and a magnification correcting unit configured to correctmagnification of at least one area of the image data separated by theimage area-separating unit.

With the configuration of the image forming apparatus according to thefirst aspect of the present invention, the magnification correction isperformed on at least one area of the image data separated by the imagearea-separating unit, and hence it is possible to accurately correct themagnification of an image without degrading the quality of the formedimage or reducing productivity, to thereby make it possible to cope withan increase in the operating speed of the apparatus.

The image area-separating unit can separate the image data into at leasttwo kinds of areas including a character portion, a photograph portion,a fine line portion, a graphic, a table, a graph, and a background.

The image area-separating unit includes a margin-determining unitconfigured to determine whether the image data is of a character portioncontaining a character edge portion and an inside of a character, or ofa margin portion not containing the character.

The magnification correcting unit can perform the magnificationcorrection on the margin portion determined by the margin-determiningunit of the image area-separating unit, by thinning lines or pixels fromthe margin portion on a line-by-line basis or a pixel-by-pixel basis orinserting lines or pixels into the margin portion on a line-by-linebasis or a pixel-by-pixel basis such that the magnification becomesequal to a specified value.

The image forming apparatus further comprises a calculation unitconfigured to calculate an expansion/contraction ratio of the recordingmedium based on a size of the recording medium before the image isheat-fixed on the recording medium, and the size of the recording mediumafter the image has been heat-fixed on the recording medium, and themagnification correcting unit performs the magnification correctionbased on the expansion/contraction ratio calculated by the calculationunit.

In a second aspect of the present invention, there is provided an imageforming apparatus that performs image formation by transferring an imageformed on an image carrier based on image data onto a recording medium,and heat-fixing the image thereon, comprising a determination unitconfigured to determine whether or not the image data exists, and amagnification correction unit configured to perform magnificationcorrection on a portion which has been determined by the determinationunit that the portion does not have the image data.

The magnification correction unit performs the magnification correctionon the portion which has been determined by the determination unit thatthe portion does not have the image data, by thinning lines or pixelsfrom the portion on a line-by-line basis or a pixel-by-pixel basis orinserting lines or pixels into the portion on a line-by-line basis or apixel-by-pixel basis such that the magnification becomes equal to aspecified value.

The image forming apparatus further comprises a calculation unitconfigured to calculate an expansion/contraction ratio of the recordingmedium based on a size of the recording medium before the image isheat-fixed on the recording medium, and the size of the recording mediumafter the image has been heat-fixed on the recording medium, and themagnification correcting unit can perform the magnification correctionbased on the expansion/contraction ratio calculated by the calculationunit.

In a third aspect of the present invention, there is provided a methodof controlling an image forming apparatus that performs image formationby transferring an image formed on an image carrier based on image dataonto a recording medium, and heat-fixing the image thereon, comprisingan image area-separating step of separating the image data into at leasttwo kinds of areas, and a magnification correction step of performingmagnification correction on at least one area of the image dataseparated by the image area-separating step.

In a fourth aspect of the present invention, there is provided a methodof controlling an image forming apparatus that performs image formationby transferring an image formed on an image carrier based on image dataonto a recording medium, and heat-fixing the image thereon, comprising adetermination step of determining whether or not the image data exists,and a magnification correction step of performing magnificationcorrection on a portion which has been determined in the determinationstep that the portion does not have the image data.

The features and advantages of the invention will become more apparentfrom the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an image processing system according to afirst embodiment of the present invention.

FIG. 2 is a block diagram of an image forming apparatus.

FIG. 3 is a block diagram of an image processing section of the imageforming apparatus, and associated parts.

FIG. 4A is a view showing an example of a bitmap image containing acharacter image.

FIG. 4B is a view of an attribute map in which attribute data items areassociated with respective pixels and arranged in two-dimension.

FIG. 5 is a view of illustrating the concept of storing five planesformed by adding a plane of the attribute map to the planes of C, M, Yand K, as an image of one page.

FIG. 6 is a view of the arrangement of data in a case in which one bitof attribute is added to each pixel.

FIG. 7 is a view of the arrangement of image data of each pixel andattribute data of the image data.

FIG. 8 is a schematic block diagram of an exposure control section of animage forming section of the image forming apparatus, and associatedparts

FIG. 9 is a block diagram of an image processing circuit in the exposurecontrol section of the image forming section of the image formingapparatus.

FIG. 10 is a flowchart of a magnification correction process executed bythe image forming apparatus.

FIG. 11A is a view which is useful in explaining the magnificationcorrection carried out when the size of an image is reduced.

FIG. 11B is a view which is useful in explaining the magnificationcorrection carried out when the size of an image is increased.

FIG. 12 is a block diagram of a variation of the image processingsection of the image forming apparatus, and associated parts.

FIG. 13 is a block diagram of an image processing section of an imageforming apparatus according to a second embodiment, and associatedparts.

FIG. 14 is a block diagram of an image processing circuit of an exposurecontrol section of an image forming section of the image formingapparatus.

FIG. 15 is a flowchart of a magnification correction process executed bythe image forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail below withreference to the accompanying drawings showing preferred embodimentthereof.

FIG. 1 is a block diagram of an image processing system according to afirst embodiment of the present invention.

Referring to FIG. 1, the image processing system is comprised of animage forming apparatus (image output apparatus) 30 including a mainunit 40 and a controller 20, and a host computer 10. The main unit 40and the controller 20 of the image forming apparatus 30 are connected toeach other by a cable 1, and the controller 20 and the host computer 10thereof are connected to each other by a cable 2. It should be notedthat any of general-purpose cables, such as parallel cables, SCSI (SmallComputer System Interface) cables, serial cables, and network cables,and dedicated cables may be used for the cables 1 and 2.

The image forming apparatus 30 is comprised of an image reading section300, an image processing section 310, an image forming section 320, anoperating section 330, and a control section 340. The controller 20 iscomprised of a CPU 21, a ROM 22, an HDD (Hard Disk Drive) controller 23,a built-in HD 24, an external interface 25, and a RAM 26. The hostcomputer 10 is an information processing unit serving as a supply sourceof print data (which is described as PDL data in the presentembodiment), and includes known component elements (a CPU, a ROM, a RAM,an HDD, a display, etc.).

The controller 20 temporarily holds PDL data, which is supplied from thehost computer 10 via the cable 2 and the external interface 25, in thebuilt-in HD 24 via the HDD controller 23. Further, the controller 20temporarily holds the PDL data held in the built-in HD 24, in a PDLbuffer 26-2 of the RAM 26 via a CPU bus 3. It should be noted that thecontroller 20 generates full-color image data or grayscale image databased on the PDL data supplied from the host computer 10.

Further, the controller 20 loads the PDL data held by the PDL buffer26-2 in a frame memory 26-1 of the RAM 26, for generation of image data.Further, the controller 20 transfers the image data loaded in the framememory 26-1 to the main unit 40 of the image forming apparatus 30, viathe cable 1. Thus, an image is formed (printed) on a recording sheet bythe image forming section 320. Further, the controller 20 is capable ofnot only obtaining various kinds of information including statusinformation of the image forming apparatus 30 via the cable 1 andtransmitting the same to the host computer 10 but also controlling theimage forming apparatus 30 based on the status information.

The CPU 21 of the controller 20 operates based on control programs, forcontrol of the functions of the controller 20. The ROM 22 stores thecontrol programs. The built-in HD 24 has a region for temporarilyholding already printed PDL data and image data generated by loading PDLdata in the frame memory 26-1, a region that stores font data, and soforth, and is connected to the CPU bus 3 via the HDD controller 23. TheRAM 26 includes the PDL buffer 26-2 for temporarily holding PDL datareceived from the host computer 10, and the frame memory 26-1 fortemporarily holding image data obtained by loading the PDL data.

It is also effective that the ROM 22 is formed e.g. by a programmablememory (e.g. EEPROM) such that control programs can be installed thereinfrom the host computer 10. Further, the ROM 22 can also be effectivelyformed e.g. by a recording medium, such as a floppy (registeredtrademark) disk or a CD-ROM, and a controller (driver) therefor. Itshould be noted that a recording medium (e.g. the ROM 22) which hascontrol programs stored therein in a state readable by the CPU 21constitutes the present invention.

Image data is transmitted from the controller 20 to the image formingapparatus 30 via the cable 1. The image forming apparatus 30 isconfigured such that it can form an image by an electrophotographicmethod, and output a full-color image. The image forming apparatus 30serves not only as a printer that forms an image on a recording sheetbased on PDL data generated by the host computer 10 but also as acopying machine that copies an image of an original on a recordingsheet, and a scanner.

The image processing section 310 of the image forming apparatus 30generates CMYK data (K data for grayscale) based on the image datasupplied from the controller 20, and supplies the CMYK data to the imageforming section 320. The image reading section 300 reads an image froman original. The image forming section 320 forms an image on a recordingsheet based on the data supplied from the image processing section 310.Further, the image forming section 320 forms an image on a recordingsheet based on the image data read from the original by the imagereading section 300. The image forming section 320 has the function ofoutputting a color image or a grayscale image having a resolution ofe.g. 400 dpi.

The operating section 330 is used when instructions are given to theimage forming apparatus 30. The control section 340 controls theoperation of the image forming apparatus 30 based on control commandsfrom the CPU 21 of the controller 20 or the instructions input from theoperating section 330. Further, the control section 340 carries out amagnification correction process described hereinafter with reference toFIG. 10, based on the control programs.

FIG. 2 is a block diagram of the image forming apparatus.

As shown in FIG. 2, the main unit 40 of the image forming apparatus 30is equipped with a DF (Document Feeder) 50 and a discharge tray 60. TheDF 50 sequentially conveys a plurality of originals stacked at apredetermined location one by one to an original reading position. Theimage reading section 300, which is formed by an optical system forscanning each original, a color CCD, and so forth, reads the originalset at the original reading position by the DF 50 or manually, generatesRGB data corresponding to an image of the original, and sends the RGBdata to the image processing section 310.

The image processing section 310 selects either of image data sent fromthe controller 20 and image data sent from the image reading section 300under the control of the control section 340, and transmits the selectedimage data to the image forming section 320 and the controller 20. Morespecifically, the control section 340 selects image data supplied fromthe controller 20 when it uses the image forming apparatus 30 as aprinter, and selects image data supplied from the image reading section300 when it uses the image forming apparatus 30 as a copying machine.

The image processing section 310 converts input RGB data into YMCK data,and supplies image data of C, M, Y and K to an exposure control section31 of the image forming section 320. The exposure control section 31converts the supplied image data of C, M, Y and K into laser beams, andcauses the laser beams to scan on a photosensitive drum 32, to therebyform a latent image thereon. A recording sheet for use in forming animage is fed by selecting an upper cassette 36 or a lower cassette 37,and is conveyed to a transfer position opposed to the photosensitivedrum 32 by way of a conveying path 4.

The latent image on the photosensitive drum 32 is developed by toner bya developing device, not shown. This forms a toner image on thephotosensitive drum 32 as a visual image, which is transferred onto thesheet. The operations for forming a latent image on the photosensitivedrum 32, developing the latent image on the photosensitive drum 32, andtransferring the toner image from the photosensitive drum 32 onto therecording sheet are performed for each of the C, M, Y, and K colors.Thus, toner images (color images) of the respective colors aresynthesized on the recording sheet to form a toner image (color image)thereon.

The recording sheet having the toner image transferred thereon isconveyed to a fixing device 33, which fixes the toner image on therecording sheet. After that, the recording sheet is discharged onto thedischarge tray 60 in a single sided recording mode for forming an imageon one side of the recording sheet. On the other hand, in a double-sidedrecording mode for forming images on both sides of the recording sheet,after termination of transfer of the toner image onto one surface of therecording sheet, the recording sheet is sent via a conveying path 5 toan inverting unit 34, where it is inverted. Then, the recording sheet isconveyed by way of a conveying path 6 and a double-sided tray 35 alongthe conveying path 4 again. The recording sheet has a toner imagetransferred onto the other surface thereof and then fixed by the fixingdevice 33, and then it is discharged onto the discharge tray 60.

FIG. 3 is a block diagram of the image processing section 310 of theimage forming apparatus, and associated parts.

As shown in FIG. 3, the image processing section 310 is comprised of adensity-luminance converting section 102, a luminance-density convertingsection 103, a smoothing circuit 104, a γ table 105, and an imagearea-separating section 107. The image signals of Y, M, C, and Kdelivered from the host computer 10 are sequentially sent to the imageprocessing section 310, each for the scanned surface of thephotosensitive drum 32 of the image forming section 320.

The density-luminance converting section 102 converts the densitysignals of Y, M, C, and K into the luminance signals of the R, G, and B(red, green, and blue) colors on a color-by-color basis by a lookuptable ROM, not shown. The luminance-density converting section 103converts the three primary color signals of the R, G, and B colorstransferred from the CCD 210 or the host computer 10 into the densitysignals of Y, M, C, and K, and delivers the density signals at apredetermined bit width (8 bits) in the order of the respective scannedsurfaces of the photosensitive drum 32.

The image area-separating section 107 includes an edge detecting section108, a chroma determining section 109, a thickness determining section110, and a lookup table (LUT) 111. The edge detecting section 108detects the edge of the image data from the image signals of Y, M, C,and K delivered from the density-luminance converting section 102, on apixel-by-pixel basis based on information from peripheral pixels, andgenerates an edge signal “edge” indicative of the edge of the imagedata, to deliver the edge signal to the LUT 111. The chroma determiningsection 109 generates a chroma signal “col” from the image signals of Y,M, C, and K delivered from the density-luminance converting section 102,and delivers the chroma signals to the LUT 111. The thicknessdetermining section 110 determines the thickness of the image data basedon the image signals of Y, M, C, and K delivered from thedensity-luminance converting section 102, and generates a thicknesssignal “zone” indicative of the thickness of the image data, to deliverthe thickness signal to the LUT 111. The LUT 111 outputs attribute dataof an image area determining signal.

The LUT 111 determines whether image data belongs to a character portion(character/graphic) including a character edge portion and the inside ofa character, a character margin portion including no character, or aphotograph portion, based on features extracted by the edge detectingsection 108, the chroma determining section 109, and the thicknessdetermining section 110. Then, the LUT 111 performs a smoothing processonly on the character portion by the smoothing circuit 104. This makesit possible to smooth the edge of the image data, whereby the imageforming section 320 can form an excellent image. It should be noted thatthe image area-separating section 107 may separate the image data notonly into character portions and photograph portions but also morefinely into fine line (ruled line) portions, graphics, tables, graphs,and a background.

Based on the results of separation of the image data into the imageareas (image area separation) by the image area-separating section 107,and area signals delivered from the operating section 330 or the hostcomputer 10, the smoothing circuit 104 carries out the followingprocess: The smoothing circuit 104 performs known image processing(switching between 400 lines and 800 lines) on a pixel-by-pixel basis,to generate data having a resolution twice as high as a readingresolution of the image read from the original. The, the y table 105converts the density data having the respective resolutions according togradation reproduction by the image forming section 320.

The image data of the Y, M, C, and K, and the attribute data generatedby the image area separation, processed as described above, are sentfrom the image processing section 310 to the exposure control section 31of the image forming section 320. When receiving the image data and theattribute data, the exposure control section 31 records densities (formsan image in accordance with the density data) by a PWM (Pulse WidthModulation) process, using a semiconductor laser, a polygon mirror, thephotosensitive drum (see FIG. 8), described hereinafter, and so forth,

FIG. 4A is a view showing an example of a bitmap image containing acharacter image, and FIG. 4B is a view of an attribute map in whichattribute data items are associated with respective pixels and arrangedin two-dimension.

FIG. 4A shows a bitmap image containing “C” as a character image. Now,let it be assumed that in the attribute map of attribute data generatedby the image area-separating section 107, pixels of the character imageare represented by 1, and pixels of the character margin portion arerepresented by 0. In this case, as shown in FIG. 4B, an attribute map isformed in which 0 and 1 associated with the respective colors arearranged in two-dimension. Further, similarly to the character pixels,photograph pixels as well have attribute data imparted thereto, and anattribute map is formed in which attribute data of character portionsand attribute data of photograph portions are integrated.

The attribute map may be configured as desired insofar as attribute dataitems can be stored in a memory, not shown, in a manner associated withrespective pixels. For example, the attribute map may be configured assuch a plane shown in FIG. 4B, and image data and the attribute map maybe separately stored in a memory. In this case, as shown in FIG. 5, fiveplanes formed by adding the plane of the attribute map to the respectiveplanes of C, M, Y and K are held as an image of one page.

Further, when the CMYK data is formed on a pixel-by-pixel basis, asshown in FIG. 7, the CMYK data may be configured such that the attributemap is embedded in a manner adding information of the attribute map tothe information of the CMYK data of each pixel. Further, to configurethe attribute map so as to prevent an increase in the amount of data,the attribute map may be configured such that it is embedded in any oneof the planes of C, M, Y and K, or lower bits of the CMYK data formed ona pixel-by-pixel basis, as shown in FIG. 6. As described above, theformat of the attribute map can be determined as required. FIG. 7 showsan example of the image data and the attribute signal formed per pixel,in which the attribute map is held by each of C, M, Y and K, and 2 bitsof attribute signal is added to 8 bits of image data of each color.

FIG. 8 is a schematic block diagram of the exposure control section 31of the image forming apparatus 320, and associated parts.

As shown in FIG. 8, the exposure control section 31 is comprised of animage processing circuit 31-1, and a laser drive section 31-2, andcorrects the magnification under the control of the control section 340.The image processing circuit 31-1 corrects the magnification in the subscanning direction based on the image data (image signals) of Y, M, C,and K, the attribute data, and magnification correction data, input fromthe image processing section 310. Further, the image processing circuit31-1 corrects the magnification of the image data in the main scanningdirection, by synchronizing the image data, which is subjected to pixeldivision modulation, with an image clock, and outputs the image data tothe laser drive section 31-2.

The laser drive section 31-2 drives the semiconductor laser 61 based onthe image data subjected to pixel division modulation and input theretofrom the image processing circuit 31-1. Within the semiconductor laser61, there is provided a photodiode sensor (hereinafter referred to as“the PD sensor”), not shown, for detecting part of the laser beams. Thelaser drive section 31-2 performs APC (Auto Power Control) of thesemiconductor laser 61 using a detection signal from the PD sensor. Alaser beams emitted from the semiconductor laser 61 is formed into asubstantially parallel or collimated beam by an optical system, notshown, including a collimator lens and a diaphragm, and enters thepolygon mirror (rotary polyhedral mirror) 62, with a predetermined beamdiameter.

The laser beam having entered the polygon mirror 62 is reflected as adeflected beam continuously changing its angle in accordance withrotation of the polygon mirror 62 rotating at a constant angularvelocity in a predetermined direction. The laser beam reflected as thedeflected beam is subjected to focusing action by an f-θ lens 63.Further, the f-θ lens 63 simultaneously corrects a distortion aberrationsuch that the time linearity of scanning is guaranteed, and thereforethe laser beam having passed through the f-θ lens 63 is focused andscanned on the photosensitive drum 32 at a constant speed in apredetermined direction.

A beam detection sensor (hereinafter referred to as “the BD sensor”),not shown, is provided in the vicinity of one end of the photosensitivedrum 32, for detecting the laser beam reflected from the polygon mirror62. A detection signal from the BD sensor is used as a synchronizationsignal for synchronizing rotation of the polygon mirror 62 with writingof data. In the laser drive section 31-2, to hold constant the amount oflaser beam during one scanning, a drive method is employed in which theoutput from a laser beam is detected by the BD sensor at a lightdetecting section during one scanning to thereby hold a drive currentfor driving the semiconductor laser 61 during the one scanning.

FIG. 9 is a block diagram of the image processing circuit 31-1 in theexposure control section 31 of the image forming section 320 of theimage forming apparatus.

As shown in FIG. 9, the image processing circuit 31-1 is comprised of amagnification correction circuit 400, a control signal generationcircuit 401, a FIFO (First In-First Out Memory) clock generation circuit405, and a parallel-serial (hereinafter referred to as “the PS”) clockgeneration circuit 407. Further, the image processing circuit 31-1includes a serial-parallel (hereinafter referred to as “the SP”) clockgeneration circuit 409, a FIFO 412, a PS conversion circuit 418, an SPconversion circuit 420, a line counter 450, a delay time generationcircuit 453, and a timing adjustment circuit 455.

The magnification correction circuit 400 corrects the magnificationbased on the image data, the attribute data, and the magnificationcorrection data, sent from the image processing section 310. In thepresent embodiment, the magnification correction circuit 400 calculatesthe expansion/contraction ratio of a recording sheet based on the sizeof the recording sheet detected by a sensor, not shown, before the tonerimage is heat-fixed thereon, and the size thereof detected after thetoner image has been heat-fixed thereon, and corrects the magnificationbased on the expansion/contraction ratio. The magnification correctiondata gives an instruction for correction (reduction correction/expansioncorrection) of the magnification of an image. In an example illustratedin FIG. 10, with reference to which the magnification correction processis described hereinafter, the magnification correction data instructs 1%of reduction correction. The same applies to the expansion correction.It should be noted that the expansion/contraction ratio of the recordingsheet may be calculated by either the control section 340 or the imageprocessing section 310.

The control signal generation circuit 401 generates a FIFO controlsignal 402 for the FIFO clock generation circuit 405, a PS conversioncontrol signal 403 for the PS clock generation circuit 407, and an SPconversion control signal 404 for the SP clock generation circuit 409.The FIFO clock generation circuit 405 generates a read clock 406 for theFIFO 412 based on a reference clock 411 and the FIFO control signal 402.

The PS clock generation circuit 407 generates a PS conversion clock 408for the PS conversion circuit 418 based on the reference clock 411 andthe PS conversion control signal 403. The SP clock generation circuit409 generates an SP conversion clock 410 for the SP conversion circuit420 based on the reference clock 411 and the SP conversion controlsignal 404. Further, the SP conversion clock 410 is output as an imageclock.

The FIFO 412 is supplied with a FIFO write address reset signal 413 anda write clock 414 from the control section 340, and receives an imagesignal indicative of each pixel and delivered from an image-generatingsection, not shown, e.g. of the host computer 10. More specifically,16-bit write pixel data 415 is input to the FIFO 412. A 16-bit readpixel data 417 is output from the FIFO 412 in response to the read clock406 input from the FIFO clock generation circuit 405 and a read addressreset signal 416 input from the control section 340. The read pixel data417 output from the FIFO 412 is input to the PS conversion circuit 418.

The PS conversion circuit 418 converts the input 16-bit read pixel data417 into a serial pixel signal 419 according to the PS conversion clock408, and outputs the serial pixel signal 419. The serial pixel signal419 is input to the SP conversion circuit 420. The SP conversion circuit420 converts the input serial pixel signal 419 into a 16-bit parallelpixel signal 421, and outputs the parallel pixel signal 421.

The line counter 450 counts a BD signal 451 output from the BD sensor,not shown, and outputs a line select signal 452 circulating in units ofe.g. 4 lines. The delay time generation circuit 453 generates four kindsof delay times 454 in synchronism with the SP conversion clock 410 inassociation with the line select signal 452. The timing adjustmentcircuit 455 causes the parallel pixel signal 421 to be delayed on aline-by-line basis according to the delay time 454, and outputs the sameas image data 456.

Next, the magnification correction executed by the image formingapparatus according to the present embodiment will be described withreference to a flowchart shown in FIG. 10.

FIG. 10 is the flowchart of the magnification correction processexecuted by the image forming apparatus.

As shown in FIG. 10, when the control section 340 of the image formingapparatus determines that the start of a printing operation has beeninstructed by an operator (step S1001), the control section 340determines whether or not front surface printing for forming an image onthe front surface of a recording sheet has been instructed (step S1002).If the control section 340 determines in the step S1002 that the frontsurface printing has been instructed, it causes the image formingsection 320 to perform image formation (front surface printing) in whicha toner image is transferred to the front surface of a recording sheet(step S1003). Further, the control section 340 causes a fixing operationto be performed for fixing the toner image on the recording sheet (stepS1004), followed by terminating the front surface printing (step S1005).

If the control section 340 determines in the step S1002 that the frontsurface printing has not been instructed, the control section 340determines whether or not back surface printing for forming an image onthe back surface of the recording sheet has been instructed (stepS1006). If the control section 340 determines in the step S1006 that theback surface printing has been instructed, it causes the exposurecontrol section 31 of the image forming section 320 to startmagnification correction (step S1007). First, the control section 340determines, on a pixel-by-pixel basis, based on image data and attributedata transmitted from the image processing section 310 whether or notthe image data belongs to a character margin portion, using the imagearea-separating section 107 (step S1008).

If the control section 340 determines in the step S1008 that the imagedata belongs to the character margin portion, the control section 340determines whether or not image data transmitted from the imageprocessing section 310 belongs to a line for magnification correction(step S1009). In the present embodiment, when the magnification iscorrected, it is assumed that 1% of reduction correction is performede.g. on image data on a A4-size sheet having a lateral length of 210 mm.Supposing that the pixel size of the image data is 600 dpi (42.3 μm), toput it simply, there are 4964 lines of data, and 49 lines correspondingto 1% of the 4964 lines are corrected.

If the control section 340 determines in the step S1009 that the currentimage data is of a line for magnification correction, the controlsection 340 causes the exposure control section 31 to thin out pixeldata to correct the magnification of the image data such that it becomesequal to a desired (specified) value (step S1010), followed byterminating the magnification correction (step S1011). The controlsection 340 repeatedly causes the exposure control section 31 to carryout the above-mentioned magnification correction. Similarly to the frontsurface printing, the control section 340 causes the image formation(back surface printing) to be performed by transferring a toner image tothe back surface of the recording sheet (step S1003), and the fixingoperation to be executed (step S1004), followed by terminating the backsurface printing (step S1005).

FIG. 11A is a view which is useful in explaining the magnificationcorrection carried out when the size of an image is reduced by thereduction correction, and FIG. 11B is a view which is useful inexplaining the magnification correction carried out when the size of animage is increased by the expansion correction.

In FIG. 11A, alphabetical portions, rectangular portions, and whiteportions represent character portions, photograph portions, andcharacter margin portions, respectively. As shown in FIG. 11A, in theimage reduction, it is possible to reduce the size of an image tocorrect the magnification thereof, by thinning out lines of thecharacter margin portions.

Similarly, in FIG. 11B, alphabetical portions, rectangular portions, andwhite portions represent character portions, photograph portions, andcharacter margin portions, respectively. As shown in FIG. 11B, in theimage expansion, it is possible to expand the size of an image tocorrect the magnification thereof, by inserting lines into the charactermargin portions.

It should be noted that although in the present embodiment, themagnification is corrected by the exposure control section 31 of theimage forming section 320, by way of example, this is not limitative,but the correction of the magnification by the image processing section310 brings about no problems. To carry out the magnification correctionprocess by the image processing section 310, as shown in FIG. 12, it isonly required to provide the magnification correction circuit 400 in theimage processing section 310. When this configuration is employed, theimage processing circuit 31-1 of the exposure control section 31 can bedispensed with. In this case, the exposure control section 31 can writean image by receiving image data from the image processing section 310to perform a predetermined process by the laser drive section 31-2.

As described hereinabove, according to the present embodiment, duringthe back surface printing, the control section 340 determines using theimage area-separating section 107 whether or not image data transmittedfrom the image processing section 310 belongs to a character marginportion. If the control section 340 determines that the image databelongs to the character margin portion, it further determines whetheror not the image data is of a line for magnification correction. Whendetermining that the image data is of a line for magnificationcorrection, the control section 340 causes the exposure control section31 to thin out pixel data to thereby correct the magnification of theimage data such that it becomes equal to a desired value. This makes itpossible to accurately correct the magnification of an imageparticularly in the sub scanning direction without degrading the qualityof the formed image, thereby making it possible to cope with increase inthe operating speed without causing degradation of productivity.

An image forming apparatus according to a second embodiment of thepresent invention is distinguished from the above-described firstembodiment in that an image processing section thereof is configured asdescribed hereinafter. The other component elements in the presentembodiment are identical to the corresponding ones in the firstembodiment (see FIGS. 1 and 2), and hence description thereof is omittedwhile designating the component elements by the same reference numerals.

In the above-described first embodiment, the method of separating imagedata into image areas and performing the magnification correctionaccording to the results of separation of the image data has beendescribed. In contrast, in the present embodiment, a description will begiven of a method of performing the magnification correction withoutseparating image data into image areas.

FIG. 13 is a schematic block diagram of the image processing section 310of the image forming apparatus according to the present embodiment, andassociated parts.

Referring to FIG. 13, the image processing section 310 is comprised ofthe density-luminance converting section 102, the luminance-densityconverting section 103, the smoothing circuit 104, the y table 105, anda determination circuit 207. The image processing section 310 accordingto the present embodiment has the same basic construction as that of theabove-described image processing section according to the firstembodiment, and hence only different points from the first embodimentwill be described.

Although in the above-mentioned first embodiment, the image processingsection 310 includes the image area-separating section 107, in thepresent embodiment, the image processing section 310 includes thedetermination circuit 207. The determination circuit 207 has a datadetermining section 208 that determines whether or not image dataexists, and sends determination data formed based on the determinationto the image forming section 320. When the image forming section 320receives the determination data, the exposure control section 31 of theimage forming section 320 performs magnification correction on portionswithout the image data (margin portions) based on the determinationdata.

FIG. 14 is a block diagram of the image processing circuit 31-1 in theexposure control section 31 of the image forming section 320 of theimage forming apparatus.

As shown in FIG. 14, the image processing circuit 31-1 is comprised ofthe magnification correction circuit 400, the control signal generationcircuit 401, the FIFO 412, the PS conversion circuit 418, the SPconversion circuit 420, the timing adjustment circuit 455, and so forth.The image processing circuit 31-1 according to the present embodimenthas the same basic construction as that of the image processing circuitof the first embodiment, and hence only different points from the firstembodiment will be described.

Although in the above-described first embodiment, image data, attributedata, and magnification correction data are sent from the imageprocessing section 310 to the image processing circuit 31-1, in thepresent embodiment, image data, determination data, and magnificationcorrection data are sent from the image processing section 310 to theimage processing circuit 31-1. The magnification correction circuit 400performs magnification correction based on the image data, thedetermination data, and the magnification correction data sent from theimage processing section 310.

Next, the magnification correction executed by the image formingapparatus according to the present embodiment will be described withreference to a flowchart shown in FIG. 15.

FIG. 15 is a flowchart of a magnification correction process executed bythe image forming apparatus.

As shown in FIG. 15, when the control section 340 of the image formingapparatus determines that the start of a printing operation has beeninstructed by an operator (step S1501), the control section 340determines whether or not front surface printing for forming an image onthe front surface of a recording sheet has been instructed (step S1502).If the control section 340 determines in the step S1502 that the frontsurface printing has been instructed, the control section 340 causes theimage forming section 320 to form an image (perform the front surfaceprinting) by transferring a toner image onto the front surface of therecording sheet (step S1503). Further, the control section 340 causes afixing operation to be performed for fixing the toner image on therecording sheet (step S1504), followed by terminating the front surfaceprinting (step S1505).

If the control section 340 determines in the step S1502 that the frontsurface printing has not been instructed, the control section 340determines whether or not back surface printing for forming an image onthe back surface of the recording sheet has been instructed (stepS1506). If the control section 340 determines in the step S1506 that theback surface printing has been instructed, the control section 340causes the exposure control section 31 of the image forming section 320to start correct magnification (step S1507). First, the control section340 determines based on image data and determination data transmittedfrom the image processing section 310, whether or not image data exists(step S1508).

If the control section 340 determines in the step S1508 that thereexists no image data (margin portions), the control section 340determines whether or not image data transmitted from the imageprocessing section 310 is of a line for magnification correction (stepS1509). In the present embodiment, when the magnification is corrected,it is assumed that 1% of reduction correction is performed e.g. on imagedata on a A4-size sheet having a lateral length of 210 mm. Supposingthat the pixel size of the image data is 600 dpi (42.3 μm), to put itsimply, there exist 4964 lines of data, and 49 lines corresponding to 1%of the 4964 lines are corrected.

If the control section 340 determines in the step S1509 that the imagedata is of a line for magnification correction, the control section 340causes the exposure control section 31 to thin out pixel data to therebycorrect the magnification of the image data such that it becomes equalto a desired value (step S1510), followed by terminating themagnification correction (step S1511). The control section 340repeatedly causes the exposure control section 31 to carry out theabove-mentioned magnification correction. Similarly to the front surfaceprinting, the control section 340 also causes the image formation (backsurface printing) to be performed by transferring a toner image onto theback surface of the recording sheet (step S1503), and the fixingoperation to be executed (step S1504), followed by terminating the backsurface printing (step S1505).

As described hereinabove, according to the present embodiment, duringthe back surface printing, the control section 340 determines by theimage area-separating section 107 whether or not image data exists. Ifthe control section 340 determines that there exists no image data, thecontrol section 340 determines whether or not the image data is of aline for magnification correction. When determining that the image datais of a line for magnification correction, the control section 340 thinsout pixel data by the exposure control section 31, and corrects themagnification of the image data such that it becomes a desired value.This makes it possible to accurately correct the magnification of animage particularly in the sub scanning direction without degrading thequality of the formed image, thereby making it possible to cope withincrease in the operating speed without causing degradation ofproductivity.

Although in the above-described first and second embodiments, themagnification is corrected by thinning or inserting lines from or intothe character margin portions, by way of example, this is notlimitative, but the magnification correction may be carried out bythinning or inserting pixels from or into the character margin portionson a pixel-by-pixel basis.

Further, although in the above-described first and second embodiments,the magnification is corrected by thinning or inserting lines from orinto the character margin portions between characters, by way ofexample, this is not limitative, but the present invention can also beapplied to a case where the magnification is corrected by thinning orinserting lines from or into margin portions (e.g. between charactersand photographs) other than the character margin portions.

Further, although in the above-described first and second embodiments,1% of reduction correction is performed as the magnification correction,by way of example, this is not limitative, but the ratio of thereduction correction can be changed as required. The same applies to theexpansion correction.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures and functions.

This application claims priority from Japanese Patent Application No.2006-318656 filed Nov. 27, 2006, which is hereby incorporated byreference herein in its entirety.

1. An image forming apparatus that performs image formation bytransferring an image formed on an image carrier based on image dataonto a recording medium, and heat-fixing the image thereon, comprising:an image area-separating unit configured to separate the image data intoat least two kinds of areas; and a magnification correcting unitconfigured to correct magnification of at least one area of the imagedata separated by said image area-separating unit.
 2. An image formingapparatus as claimed in claim 1, wherein said image area-separating unitseparates the image data into at least two kinds of areas including acharacter portion, a photograph portion, a fine line portion, a graphic,a table, a graph, and a background.
 3. An image forming apparatus asclaimed in claim 1, wherein said image area-separating unit includes amargin-determining unit configured to determine whether the image datais of a character portion containing a character edge portion and aninside of a character, or of a margin portion not containing thecharacter.
 4. An image forming apparatus as claimed in claim 3, whereinsaid magnification correcting unit performs the magnification correctionon the margin portion determined by said margin-determining unit of saidimage area-separating unit, by thinning lines or pixels from the marginportion on a line-by-line basis or a pixel-by-pixel basis or insertinglines or pixels into the margin portion on a line-by-line basis or apixel-by-pixel basis such that the magnification becomes equal to aspecified value.
 5. An image forming apparatus as claimed in claim 1,further comprising a calculation unit configured to calculate anexpansion/contraction ratio of the recording medium based on a size ofthe recording medium before the image is heat-fixed on the recordingmedium, and the size of the recording medium after the image has beenheat-fixed on the recording medium, and wherein said magnificationcorrecting unit performs the magnification correction based on theexpansion/contraction ratio calculated by said calculation unit.
 6. Animage forming apparatus that performs image formation by transferring animage formed on an image carrier based on image data onto a recordingmedium, and heat-fixing the image thereon, comprising: a determinationunit configured to determine whether or not the image data exists; and amagnification correction unit configured to perform magnificationcorrection on a portion which has been determined by said determinationunit that the portion does not have the image data.
 7. An image formingapparatus as claimed in claim 6, wherein said magnification correctionunit performs the magnification correction on the portion which has beendetermined by said determination unit that the portion does not have theimage data, by thinning lines or pixels from the portion on aline-by-line basis or a pixel-by-pixel basis or inserting lines orpixels into the portion on a line-by-line basis or a pixel-by-pixelbasis such that the magnification becomes equal to a specified value. 8.An image forming apparatus as claimed in claim 6, further comprising acalculation unit configured to calculate an expansion/contraction ratioof the recording medium based on a size of the recording medium beforethe image is heat-fixed on the recording medium, and the size of therecording medium after the image has been heat-fixed on the recordingmedium, and wherein said magnification correcting unit performs themagnification correction based on the expansion/contraction ratiocalculated by said calculation unit.
 9. A method of controlling an imageforming apparatus that performs image formation by transferring an imageformed on an image carrier based on image data onto a recording medium,and heat-fixing the image thereon, comprising: an image area-separatingstep of separating the image data into at least two kinds of areas; anda magnification correction step of performing magnification correctionon at least one area of the image data separated by said imagearea-separating step.
 10. A method of controlling an image formingapparatus that performs image formation by transferring an image formedon an image carrier based on image data onto a recording medium, andheat-fixing the image thereon, comprising: a determination step ofdetermining whether or not the image data exists; and a magnificationcorrection step of performing magnification correction on a portionwhich has been determined in said determination step that the portiondoes not have the image data.